What is the pH of a solution prepared by mixing 50.0 mL of 0.30 M #HF# with 50.00 mL of 0.030 M #NaF#?

Assume that the volume of the solution are additive and that Ka= #1.72* 10^-4# for #HF#?

1 Answer
Jul 17, 2016


This is a buffer solution. To solve, you use the Henderson Hasselbalch equation.


#pH = pKa+log ([conj. base]/[acid])#

The HF is the weak acid and its conjugate base is NaF. You are given the Molar and Volume of each. Since you are changing the volume, your molarity changes as well. To find the moles of the conj base and acid, first find the moles with the given Molar and Volume and then divide by the total Volume of the solution to find your new Molar concentration.

Conceptually speaking, you have 10x more acid than base. This means you have a ratio of 1:10. Your answer should reflect a more acidic solution.

The pKa can be found by taking the -log of the Ka. After finding your pKa, you subtract by 1 after finding the log of the ratio and that is the pH of the solution.